This invention relates to an apparatus for detecting malfunction of a speed indication device such as a tachometer which is provided mainly for an outboard motor or the like.
A conventional malfunction detection apparatus will be described with reference to FIG. 1 showing a circuit diagram therefor. Published Unexamined Japanese Patent Application No. Hei. 2-102374 discloses an example of such a conventional malfunction detection apparatus as shown in FIG. 1.
The conventional malfunction detection apparatus shown in FIG. 1 includes a 12 V battery 1, a generator 2 provided for an outboard engine or motor and a tachometer output circuit 3 the input and output sides of which are connected to the generator 2 and a tachometer 4, respectively.
The conventional apparatus thus organized operates as follows.
The tachometer output circuit 3 subjects the output signal of the generator 2 to signal processing such as waveform shaping to provide a tachometer signal indicative of the rotation speed (rpm) of the engine or motor. The tachometer signal thus provided is applied to the tachometer 4.
With such an outboard engine or motor system as described above, because such engine or motor is high in vibration and the tachometer is large in power consumption, there often occurs malfunctions or problems in the outboard engine system. As mentioned above, it is true that the tachometer is large in power consumption; however, the main cause of the malfunction or problem is not the mechanism of the tachometer per se but the disconnection of the battery due to the vibration, or the voltage drop in the battery due to discharge.
Accordingly, even when the tachometer operates erroneously for instance because the battery has been disconnected unintentionally, there may be a possibility that the operator may determine that the tachometer per se is out of order, and should be replaced by new one resulting in prolong a period of time for find out the true causes.